This invention relates to a lead bearing electrode(s) adapted to be connected to an electrical device and to contact a living organ to receive or conduct electrical signals there between. Notwithstanding its various uses, this invention will be described as an endocardial pacing and sensing lead for connecting a cardiac pacemaker pulse generator to cardiac tissue.
Body implantable cardiac pacemaker leads are quite well known, their construction and function being described at length in medical journals during the past 15 years. The standard endocardial lead is of the type shown is U.S. Pat. No. 3,348,548, for example, and comprises lengths of coiled wire conductors extending between a proximal pin(s) adapted to be connected to a pulse generator and a distal electrode(s) adapted to contact the endocardium of the heart. The lead is inserted and guided through a selected vein of the body until the distal end thereof is lodged in the apex of the right ventricle of the heart. A material, such as silicone rubber, that is both electrical insulating and impervious to body fluids and tissue encases the coiled wire conductor, either by a molding process or by insertion of the coiled wire conductor in a length of hollow silicone rubber tubing. In either case, a lumen extends down the center of the coiled wire conductor into which a stylet is advanced prior to insertion of the lead into the patient's vein to advance the lead through the patient's vein and to place the distal end of the lead bearing the electrode(s) at the desired position in the patient's heart.
Coiled wire conductor endocardial leads of the type disclosed in the aforementioned U.S. Pat. No. 3,348,548 possess a first flexibility and, when attached at their pins to a pulse generator, possess an axial mechanical loading force transmitted from the point of immobilization of the pulse generator down the length of the lead to its distal end. In chronic use of such leads, it has been observed that the contractions of the heart against the distal end of the lead and the axial mechanical force applied thereto may traumatize the tissue in contact with the distal end. The traumatized tissue may form a ball of scar (fibrous) tissue around the electrode(s) to reduce the stress placed upon the endocardium below the tissue damage limit. In chronic experience, it is observed that the threshold currents sufficient to stimulate the heart increase as the scar tissue is formed until the thickness of the scar tissue stabilizes. The required energy to stimulate the heart is thereby increased because of the additional potential drop across the nonresponsive scar tissue between the electrode surface and responsive cardiac cells. A discussion of this mechanism in conjunction with the realization of optimum electrode surface area appears in the paper entitled "Comparison of Power Sources for Advanced Pacemaker Applications" by Rasor, Spickler and Clabaugh (procedings of the Seventh Intersociety Energy Conversion Engineering Conference, January, 1972, pages 752-760).
However, it remains the consensus that the coiled wire conductor is necessary to restrain the stiffening stylet from perforating the silicone rubber encasing material at any point along the length of the lead where it departs radically from a straight line. In the implantation procedure, once the physician has advanced the distal end of the lead into the right ventricle, he may find it necessary to withdraw the stylet, form a curve at the distal end thereof, and reinsert the stylet into the lumen of the lead down to the distal end thereof. If the side wall of the lead is not sufficiently strong, the stylet, especially with the curvature in the distal end thereof, may well pass through the side wall of the lead and snag in the vein. The article "Technique for Insertion of Transvenous Endocardial Pacemakers" by N. P. D. Smyth, M.D. (Journal of Thoracic and Cardiovascular Surgery, vol. 51, pp 755-758, 1966) depicts the tortuous transvenous path of endocardial leads.
It has been recognized that the flexibility of the coiled wire conductor may be increased by reducing its diameter. However, reduction in the diameter increases the wire's electrical resistance and the resulting waste of energy is unacceptable.
It has also been know that there are other advantageous and flexible lead conductor configurations. For example, U.S. Pat. No. 3,572,344 discloses a lead construction commonly used in myocardial leads, where stylets are unnecessary, commonly referred to as the "tinsel wire" lead. The tinsel wire lead conductor has a nonconductive fiber core around which are wrapped a plurality of conductor strands. Each strand comprises an electrical conductor strip itself wrapped around a separate nonconductive fiber core. The strands and strips are helically wound about the main core and the windings are spaced one from the other. Leads constructed of such tinsel wire are notably free from axial mechanical loading forces and are resistant to fracture. Such leads may retain their strength and durability and still be manufactured in a diameter somewhat smaller than that typically used in coiled wire construction leads. Unfortunately, the tinsel wire construction is incapable of restraining a stylet from passing through the side wall of the lead. No advantage is known to obtain over coiled wire conductor leads by making the tinsel wire lead strong enough to withstand such puncture, either by increasing the number of tinsel wire conductor strand, their thickness or other properties or by adding a puncture-resistant nonconductive tube for the lumen to the construction of the lead.
The body implantable lead of the present invention combines all the advantages of both types of lead construction with none of the attendant disadvantages set forth above. One of the features of the present invention is the provision of a puncture-resistant lumen through that portion of the lead that may be susceptible to puncture and the elimination of the axial mechanical load of the lead on the endocardium. Another feature of the invention is that the conductors used in the lead comprise reliable designs that enjoy demonstrated reliability in chronic use permitting easy placement of the lead in the heart according to well-known techniques.